Dendritic spines host >90 percent of excitatory synapses; they are lost or have abnormal structure in many developmental disorders that disrupt central nervous system function. The overall goal is to understand the role of spine and synapse structure in the normal development of learning and memory. Long-term potentiation (LTP) is a synaptic model of learning and memory well-suited to investigate this process. Spines are thought to be important because they sequester core structures and molecules needed for the protein synthesis-dependent or late phase of LTP (L-LTP) lasting >3hr. A clear understanding requires the nanometer resolution of 3D reconstruction from serial section electron microscopy, an approach pioneered in this laboratory. Rigorous experiments are proposed to test whether formation of dendritic spines and structural synaptic plasticity provide general mechanisms for the developmental regulation of L-LTP in hippocampus, a brain region crucial for learning and memory. Aim 1 is to test the hypothesis that the abrupt onset of L-LTP at postnatal day (P)12 is associated with first occurrence of dendritic spines and capacity for structural synaptic plasticity. The experiments will determine what differentiates dendritic, axonal, spine, and synaptic structure and composition at P12, from P8 and P10 when L-LTP is not produced by one bout of TBS. They will test whether production of L-LTP at P12 results in a balanced elimination of small spines and enlargement of remaining synapses as occurs in mature hippocampus and whether pre- and postsynaptic structural remodeling are synchronized during development with the ability to express L-LTP. Aim 2 is to test the hypothesis that dendritic spines are induced by TBS and then serve to sustain L-LTP after a second bout of TBS at P10, but not at P8, when multiple TBS do not produce L-LTP. Aim 3 is to ascertain the developmental onset of L-LTP and its ultrastructural correlates in mouse hippocampus as a foundation for future work using genetic manipulations. The outcomes promise new insight into the synaptic basis of learning and memory, essential knowledge to design effective treatments for developmental brain disorders.